The purpose of the research in this laboratory is to investigate chemical and biochemical factors which account for 'false positive' mutagenic noncarcinogens. This investigation of nonconcordance between the short- term genotoxicity assays and the results of bioassays has determined that the mutagenic carcinogen-noncarcinogen pair 2,4-& 2,6-diaminotoluene (DAT) differ only in that the carcinogenic isomer produced a dose-related increase in cell proliferation in the liver whereas the noncarcinogen produced no increase in cell proliferation even at higher doses. Subsequent studies utilizing the incorporation of bromodeoxyuridine into newly synthesized DNA have demonstrated that the mutagenic noncarcinogen- carcinogen pair 1-& 2-nitropropane (NP) have a similar pattern of toxicity. That is, the hepatocarcinogenic isomer 2-NP produced a dose-related increase in cell proliferation in the liver yet the noncarcinogenic isomer 1-NP did not. These results suggest and further confirm a positive correlation between cell proliferation and carcinogenicity for these chemicals, irrespective of their mutagenicity. Future research will expand these findings with other chemical pairs, especially mutagenic carcinogen- noncarcinogen pairs whenever possible to further evaluate the role of cell proliferation in experimental carcinogenesis.